The present invention is generally directed to the manufacture of a semiconductor device. In particular, the present invention relates to a process that provides for enhancement of contact sidewall coverage of refractory metal nitrides.
Thin film aluminum and aluminum alloys are fundamental materials having application in semiconductor processing. Aluminum is a good conductor, adheres well to silicon and silicon dioxide. A significant problem in using aluminum for interconnects stems from problems of junction spiking that may occur at the interface of pure aluminum and silicon when the interface is heated as is with normal semiconductor processing heat treatments (e.g., annealing). This spiking occurs between the aluminum/silicon interface of a the p-n junction below a contact owing to the solubility of silicon in aluminum (e.g., to about 0.5% at 400xc2x0 C.) which increases with temperature. If the penetration by the aluminum is beyond the p-n junction depth below the contact, the junction will be electrically shorted.
One technique to address this challenge is to use an aluminum alloy having a concentration of silicon in excess of the silicon solubility at the maximum process temperature the substrate may encounter. Thus, when the (Alxe2x80x94Si)-silicon interface is heated, the aluminum alloy film does not draw silicon into solution from the substrate, and junction spiking is avoided.
As device dimensions are approaching fractional microns, the use of Alxe2x80x94Si films appears to be inadequate. The shallower junctions, narrower metal lines, and more severe topography of VLSI circuits are less tolerant of Si precipitation in the Alxe2x80x94Si film that occurs when such films are cooled down from 400xc2x0 C. temperature steps. In addition, electromigration failure at Alxe2x80x94Si contacts becomes- more severe as the contacts get smaller. In an effort to address the shortcomings of Alxe2x80x94Si contacts, more elaborate contact structures employing barrier materials and other layers have been used as replacements (e.g., Si/W/Al, or Si/PtSi/TiW/Al contact structures.
Refractory metal nitrides, particularly titanium nitride TiN) are employed as a barrier material to prevent spiking of aluminum into a silicon substrate. The barrier material is often applied by chemical vapor deposition (CVD) or sputtering. In one example process, titanium nitride (TiN) is used as a barrier material in conjunction with tungsten prior to the deposition of aluminum. TiN is a thin-film of choice for CVD tungsten (W) adhesion layer. TiN is also used as a fluorine-resistant glue layer during subsequent tungsten CVD where WF6 is a reactant. Since tungsten does not stick to oxide surface underneath, peeling may occur wherever there is not a TiN film present. In addition, the reaction of WF6 to any exposed oxide layer causes a chemical reaction that manifests itself in the production of xe2x80x9cvolcano-likexe2x80x9d damage at the top corners of the contacts. Consequently, having good TiN layer coverage inside the contact is important for device reliability and yield.
Sputtering is a widely used deposition technique for a variety of metallic films, including aluminum, aluminum alloys, platinum, gold, titanium-tungsten, tungsten. Sputtering describes the mechanism in which atoms are dislodged from the surface of a material by collision with high-energy particles. Ionized metal plasma (IMP) sputtering provides good contact bottom coverage of TiN layer for small feature sized devices of less than 0.25 xcexcm. However, the sidewall coverage is not sufficient and may result in the aforementioned shortcomings. In U.S. Pat. No. 4,783,248 titled xe2x80x9cMethod for the Production of a Titanium/Titanium Nitride Double Layer,xe2x80x9d issued to Armin Kohlhase et al., discusses some of the challenges involved in producing a titanium-based film and is incorporated herein by reference in its entirety.
Refer to FIG. 1. In a conventional process on a silicon substrate 100, a poly silicon or a diffusion layer 110 has been subjected to a silicidation 120 with a refractory metal, usually titanium. An inter-metal oxide (IMO) layer 130 provides electrical isolation for the contact region 150 defined in the IMO layer 130. A layer 140 of TiN is sputtered on the contact region 150 and exhibits thinning at 140a. If too thin, there may be a lack of coverage at 140a. 
There exists a need to provide overall sufficient coverage in the contacts so that device reliability is maintained at a reasonable cost.
The present invention is exemplified in a number of implementations, one of which is summarized below. During the forming of electrical contacts in a device structure, it is a challenge to ensure that metal will sufficiently adhere to the dielectric materials surrounding the contacts. Tungsten does not stick to an oxide surface. Consequently, an adhesion layer is provided to improve the bond and to protect the underlying oxide layer from reacting with byproducts of the tungsten deposition process. Usually, such an adhesion layer is titanium nitride. In forming the adhesion layer, coverage on the bottom and sidewalls of the contact has to be assured to minimize contact failure. In an example embodiment according to the present invention, on a semiconductor substrate, there is a method for depositing an adhesion layer in a contact region having a bottom and sidewalls. The method comprises depositing a first coat of the adhesion layer in the contact regions so that the bottom receives the first coat of adhesion layer at thickness greater than the thickness on at least part of the sidewalls. Next, a second coat of the adhesion layer in the contact region is deposited so that the sidewalls receive the second coat of adhesion layer at a thickness greater than the thickness greater than the thickness on the bottom.
The above summary of the present invention is not intended to represent each disclosed embodiment, or every aspect, of the present invention. Other aspects and example embodiments are provided in the figures and the detailed description that follow.